This invention relates to a process and an apparatus for producing evaporated phosphor sheets and more particularly to a process and an apparatus for producing evaporated phosphor sheets with a protective measure being so taken that the heat of radiation from sections of evaporating film forming materials will affect neither the substrate on which a film is being deposited nor any other parts, thereby ensuring that the phosphor sheet as produced will not have been subjected to thermal history. The invention also relates to the evaporated phosphor sheet thus obtained by means of such process and apparatus. There are known a class of phosphors which accumulate a portion of applied radiations (e.g. x-rays, α-rays, β-rays, γ-rays, electron beams and uv radiation) and which, upon stimulation by exciting light such as visible light, give off photostimulated luminescence in proportion to the accumulated energy of radiation. Such phosphors, called stimulable phosphors, are employed in medical and various other applications.
An exemplary application is a radiation image information recording and reproducing system which employs a stimulable phosphor sheet having a layer formed of the stimulable phosphor. The layer is hereunder referred to as the phosphor layer and the stimulable phosphor sheet is hereunder referred to simply as a phosphor sheet. The phosphor sheet is sometimes called a radiation image transforming panel but in the following description, the term “phosphor sheet” is used throughout. The radiation image information recording and reproducing system has already been commercialized by various companies including Fuji Photo Film Co., Ltd. which has marketed FCR (Fuji Computed Radiography, trade name).
In the system, radiation image information about the subject such as the human body is recorded on the phosphor sheet (more specifically, the phosphor layer) and thereafter the phosphor sheet is scanned two-dimensionally with exciting light such as laser light to produce photostimulated luminescence which, in turn, is read photoelectrically to yield an image signal and an image reproduced on the basis of the image signal is output as a visible image, typically on a recording material such as a photosensitive material or to a display device such as CRT. After reading has been finished, the residual image is erased from the phosphor sheet for subsequent cyclic use.
The phosphor sheet is typically produced by a process comprising the steps of preparing a coating solution having the particles of a stimulable phosphor dispersed in a solvent containing a binder, etc., applying the coating solution to a support in sheet form that is made of glass, resin, etc. and drying the applied coating to form a phosphor layer.
Phosphor sheets are also known that are made by forming a phosphor layer on a support through methods of physical vapor deposition (vapor-phase film formation) such as vacuum evaporation and sputtering.
The phosphor layer formed on the support by physical vapor deposition has excellent characteristics. First, it contains less impurities since it is formed under vacuum; in addition, it is substantially free of any substances other than the stimulable phosphor, as exemplified by the binder, so it has high uniformity in performance and still assures very high luminous efficiency.
Vacuum evaporation involves evaporating film forming materials from evaporation sources in a vacuum chamber and depositing a phosphor layer on a substrate's surface.
The phosphor layer of the phosphor sheet is very thick, ranging from about 200 μm through a typical value of about 500 μm up to sometimes more than 1000 μm. In order to ensure that such thick phosphor layers are deposited uniformly, columns of good shape need to be formed by adopting the approach of introducing a gas and, in addition, uniform film thickness must be assured by thickness measurement so as to provide better x-ray characteristics.
A problem with the approach of introducing a gas technique is that the percentage of the evaporated particles of the film forming materials that arrive at the substrate (i.e., the percent deposition on the substrate) decreases as the number of gas molecules in the vacuum chamber increases. This difficulty may be dealt with by shortening the distance between the substrate and the source of vaporization of the film forming material within the vacuum chamber (namely, by bringing the two members closer to each other than in the prior art).
However, this approach of shortening the distance between the substrate and the source of vaporization of the film forming material gives rise to another problem that has been absent from the prior art, that is, the heat of radiation from the evaporation source causes excessive heating of the substrate. As is well known, the x-ray characteristics of stimulable phosphors (and phosphor sheets employing them) can potentially deteriorate if they are subjected to thermal history, so excessive heating of the substrate is by no means preferred.